The invention described herein relates to the monitoring of movable reflectors in optical switches. In particular, the invention relates to methods and apparatus for internally monitoring and controlling the orientation of movable reflectors in an optical switch. Still more particularly, some embodiments of the invention relate to methods and apparatus for using internal auxiliary monitor light beams to determine the position and orientation of movable reflectors of the optical switch so that the movable reflectors can be positioned and maintained at a desired orientation.
In recent years there have been extensive efforts to develop commercially fill viable optical switches. Presently there are a variety of different types of optical switch architectures available on the market. One proposed optical switch architecture contemplates the use of arrays of Micro Electro-Mechanical Systems (MEMS) mirrors to accomplish the switching. Such an optical switching system is described in International (PCT) Publication Number WO99/66354, naming Herzel Laor as inventor, and entitled, xe2x80x9cPLANAR ARRAY OPTICAL SWITCH AND METHOD,xe2x80x9d published on Dec. 23, 1999, the entirety of which is incorporated herein by reference for all purposes. A perceived advantage of this type of optical switching system is that it is potentially scalable to many channels.
An example of a conventional MEMS mirror-based optical switching system is diagrammatically represented in FIG. 1A of the drawings. In the embodiment shown in FIG. 1A, the optical switch 100 includes an input fiber array 102 an input lens array 104 input and output reflector arrays 106, 108, an output lens array 110 and an output fiber array 112. According to the configuration of FIG. 1A, multiple optical inputs are used with an equal number of lenses to produce an equal number of approximately collimated input optical beams. The input and output mirror arrays 106, 108 each include a plurality of movable mirrors, such as mirror 106a. In the depicted embodiment, each movable mirror in the input and output mirror arrays is rotatable about two orthogonal axes so that an input beam received on any one of the input fibers can be directed towards a plurality of the output fibers by appropriately adjusting the orientation of their associated mirrors. The depicted embodiment is shown with an input optical beam 101a being directed through the switch into a selected output fiber as an output beam 101b. By changing the position of movable mirrors in the reflector arrays 106, 108 the input optical beams (e.g., 101a) can be switched to one of a plurality of output fibers as an output beam (e.g., 101b). The input and output fiber blocks 102, 112 are typically comprised of a two-dimensional array of fibers with polished end faces. The input fiber block 102 is positioned adjacent an input lens array 104 to provide collimated input beams, while the output lens array 110 is positioned adjacent to couple collimated output beams into output fiber block 112. The input and output mirror arrays 106, 108 each include a plurality of movable mirrors, such as mirror 106a. Although the depicted embodiment has movable mirrors configured to rotate in two axes, other embodiments have movable mirrors configured so that they are rotatable about a single axis.
In theory, the mirror arrays can be formed using a wide variety of techniques, and different companies have adopted different approaches in their attempts to provide suitable mirror arrays. By way of example, one approach is to create movable mirrors by forming MEMS structures on a monolithic silicon substrate. Devices such as these are commercially available from a variety of sources, including MCNC of Research Triangle Park, N.C. and Analog Devices of Cambridge, Mass.
An alternate embodiment 150 of a conventional MEMS mirror-based optical switching system is shown in FIG. 1B of the drawings. In this configuration, a xe2x80x9cfoldedxe2x80x9d optical path is provided by using a fixed mirror 158 that cooperates with a movable mirror array 106 so that an input beam 101a from fiber array 102 passes through lens array 104 to an input mirror 106a in movable mirror array 106. The input beam 101a is reflected off a first movable mirror 106a and directed to the fixed mirror 158, whereupon the beam is then reflected off of the fixed mirror 158 to an appropriate second movable mirror 106b which directs the beam (as output beam 101b) through the lens array 104 to a desired output channel in the fiber array 102. Thus, it will be appreciated that fiber array 102 may operate as both the input array of optical fibers and the output array of optical fibers. Thus, the fiber array 102 can include an input fiber array portion and an output fiber array portion.
In general, the movable mirror arrays 106, 112 of FIGS. 1A and 1B are populated with as many mirrors as fibers in the input/output fiber array; however, only two mirrors are shown for clarity. The mirrored configuration shown in FIG. 1B has the advantage that, in principle, any fiber can be switched to any other fiber, and so fibers do not have to be divided into sets of input fibers and output fibers.
By adjusting mirror position, optical beams can be steered from selected input fibers to selected output fibers. By monitoring and precisely adjusting the position of the movable mirrors, an optical beam from an input fiber can be switched to one of a plurality of output fibers, thereby accomplishing optical switching.
Another approach to achieving optical beam switching and mirror position control using monitor xe2x80x9ctapsxe2x80x9d to measure a portion of optical beam power using such measurements to adjust mirror position. FIG. 2 depicts one such approach. FIG. 2 shows a plurality of input fibers 1 feeding input signals into an optical switching apparatus 5. The switch directs the signals to the desired output fibers 4 where they can be transmitted through the system. The optical power of the input signals is tapped from the input fibers 1 by fiber taps that direct a portion of the signal into detectors 2 where the optical power is measured. Similarly, output optical power is tapped from the output fibers 4 and detected by detectors 3 where the optical power is measured. These measurements are compared, and using loss optimization techniques the position of movable mirrors in the switch 5 is adjusted to produce output beams of a desired power. However, such switches require complex signal processing algorithms and require the use of many expensive optical taps and optical detectors, thereby driving up the cost.
FIG. 3 is a block diagram of yet another approach. This approach is disclosed in the International Patent WO 99/67666 to H. Laor, which is hereby incorporated by reference. In the depicted optical switch, an optical signal 29 is input through an input fiber 21. The input signal 29 is directed onto a first movable mirror 25a that steers the signal 29 onto the second movable mirror 25b, which reflects the signal 29 into the desired output fiber 24. Additional components are used to monitor the mirror orientation (position). Arranged along the signal 29 path are a first beam splitter 31, a second beam splitter 32, a third beam splitter 33, a fourth beam splitter 34, a first lens 26a, and a second lens 26b. Also included are a first laser 27, a second laser 28, a first detector 22, and a second detector 23. The first laser 27 generates a forward propagating laser beam that is directed along the optical path toward the output fiber 24. After passing through a plurality of beam splitters 32, 33, lens 26b, and both mirrors 25a, 25b, the beam passes through the fourth beam splitter 34, where a portion of the beam is directed into the second detector 23, where it is measured. Similarly, a beam generated by the second laser 28 is back propagated toward the input fiber 21. A portion of this beam is split by first beam splitter 31, which directs a portion of the beam into the first detector 22, where it is measured. These two measurements are used to determine if the movable mirrors 25a, 25b are correctly oriented. The large number of lasers, beam splitters, and detectors in this device can be cost prohibitive.
These attempts to solve the problem of monitoring and controlling movable mirror position forgoing problems have met with mixed success. Such conventional switches are expensive, difficult to construct, and suffer from reduced optical throughput. What is needed is a lower cost apparatus and technique for monitoring the position of the movable mirrors in an optical switch.
In accordance with the principles of the present invention, an apparatus and method for monitoring and adjusting the position of movable mirrors in an optical switch is disclosed.
One embodiment discloses a diagnostic device for detecting the alignment of movable reflectors in an optical switch. The device can be incorporated into an optical switch that includes optical input fibers, optical output fibers, and an array of movable mirrors. Such mirrors are capable of occupying a plurality of positions. The movable mirrors are configured to steer input light beams received from the array of input fibers along a signal path so that each input light beam can be switched to one of a plurality of output fibers as an output beam. The diagnostic device embodiment includes a two-dimensional photoimager positioned to receive light from the reflector array. Each movable mirror of the reflector array reflects light to different two-dimensional positions on the photoimager based on the current position of each movable mirror, thereby creating a two-dimensional image of the reflector array. A controller receives information from the photoimager and adjusts the positions of the movable mirrors of the reflector array according to light received at the photoimager.
In another embodiment, an optical switch includes an array of optical input fibers configured to carry input light beams and an array of output fibers. The switch includes a switching element including a reflector array having movable mirrors. Each movable mirror is adjustable to a plurality of positions suitable for reflecting selected input light beams received from selected input fibers into selected output fibers, enabling the switching of each input light beam along a signal path to one of a plurality of output fibers as an output beam. The switch also includes a diagnostic device having an illumination source for directing monitor light beams onto the movable mirrors of the reflector array where the monitor beams are reflected onto a photoimager. The photoimager is arranged to receive monitor light beams reflected from the movable mirrors, with each movable mirror of the reflector array reflecting light onto a different two-dimensional position on the photoimager depending on which one of the plurality of positions each mirror currently occupies. This configuration provides two-dimensional information concerning the current position of the movable mirrors. The switch also includes a controller for adjusting the position of the movable mirrors of the reflector array according to light received at the photoimager.
Another aspect of the invention includes an anti-reflective coating embodiment. The anti-reflective coating can receive light incident at angles ranging from about 5 degrees to about 55 degrees, and transmit the light in a first bandpass region for transmitting light having a wavelength of less than 1 xcexcm and a second bandpass region for transmitting light having a wavelength of greater than 1 xcexcm.
Yet another embodiment includes a method for detecting whether movable mirrors in an optical switch have a desired orientation. The method comprises the steps of directing at least one light beam onto movable mirrors of a reflector array in an optical switch and receiving a light beam reflected from the movable mirrors of the reflector array. The two-dimensional position of the at least one reflected light beam is detected and compared to a two-dimensional position of a reflected light beam having a desired two-dimensional position that corresponds to that of a movable mirror having the desired orientation. Using this information, a determination is made as to whether the movable mirror is positioned at the desired orientation. If the mirror does not occupy the desired position, the method adjusts the mirror until the desired position is attained.
These and other aspects and advantages of the invention will become apparent from the following detailed description and accompanying drawings which illustrate, by way of example, the principles of the invention.